Arguments:

Control signal amplitude threshold, which determines the break point between slopeBelow and slopeAbove. Usually 0..1. The control signal amplitude is calculated using RMS.

slopeBelow

Slope of the amplitude curve below the threshold. If this slope > 1.0, the amplitude will drop off more quickly the softer the control signal gets; when the control signal is close to 0 amplitude, the output should be exactly zero -- hence, noise gating. Values < 1.0 are possible, but it means that a very low-level control signal will cause the input signal to be amplified, which would raise the noise floor.

slopeAbove

Same thing, but above the threshold. Values < 1.0 achieve compression (louder signals are attenuated); > 1.0, you get expansion (louder signals are made even louder). For 3:1 compression, you would use a value of 1/3 here.

clampTime

The amount of time it takes for the amplitude adjustment to kick in fully. This is usually pretty small, not much more than 10 milliseconds (the default value). I often set it as low as 2 milliseconds (0.002).

relaxTime

The amount of time for the amplitude adjustment to be released. Usually a bit longer than clampTime; if both times are too short, you can get some (possibly unwanted) artifacts.